The present invention relates generally to an improved composite material for use as an interface or in forming a mounting pad to be interposed along a heat dissipating path between a solid state electronic device and a heat sinking surface. The composite consists of a multi-layer pad flanked by skin layers which may be different in chemical composition and/or physical properties integrally bonded to and mated with the bulk layer so as to form a composite having good mechanical properties along with excellent thermal properties. The composites of the present invention are useful in the production and manufacturing of electronic systems inasmuch as the skin layers are well adapted to receive and reasonably hold a release liner, which is readily removable from the composite during or following production and handling operations.
In order to enhance the thermal properties of interface mounting pads, it is generally the practice to increase the particulate loading of the resin system of a composite. Increased loading, while improving the thermal properties, generally has an adverse effect upon the mechanical properties. To decrease hardness of highly filled composites, it is customary to reduce cross-linking of the resin system or use gel-like resins. In this connection, excessive loading and light cross-linking typically leads to a reduction in the cohesive strength of the blend, so that the product becomes difficult and/or impossible to handle in production operations. By way of example, the effective use of a release film is impeded because of the tendency for portions of the mounting pad to adhere to the release film upon removal. Also, the pad can tear while it is being pulled a liner due to its low cohesive strength. Loss of portions of the pad renders the structure unusable for its intended purpose due to the creation of air entraining voids. Surface disruptions of any sort are, of course, unacceptable for interface mounting pads. The present invention aims to produce a pad that is soft, highly filled and can be easily processed in a production operation.
The bulk layer may comprise a silicone elastomer containing thermally conductive fillers such as, for example, alumina, boron nitride, aluminum nitride, graphite, zinc oxide, aluminum, copper powder, silver powder and other thermally conductive ceramics or metals including blends or mixtures thereof. As indicated above, however, the bulk layer employs a high concentration of such fillers. Alternative resins for the bulk and surface layers that may be useful are polyethylene, epoxy, acrylic, polyurethane, polyester, or polybutadiene. The bulk layer has a thermal conductivity in the range of 1-15 W/m.K. with a hardness ranging from between about 10-80 Shore 00. The thickness of the bulk layer will typically range from between about 10 and 250 mils.
The surface or skin layers are likewise filled with similar conductive fillers. The skin layer may employ the same resin system as the bulk layer, with distinctively different mechanical properties. Other resin systems including epoxy, acrylic, polyurethane, polyester or elastomeric rubbers compatible with that of the bulk may also be employed. The skin layer typically has a thermal conductivity ranging from between about 0.3-5 W/m.K. with a significantly higher hardness than that of the bulk, such as in the range of between about 30 Shore 00 to 60 Shore A. The properties of the skin layer permit it to be applied as a hot-melt of flexible film, thereby not adversely affecting the overall thermal performance of the bulk or center layer. Furthermore, it neither alters nor increases the hardness of the bulk material inasmuch as the skin layer is typically between about 2 and 50 microns in thickness.
The thermal interface pads of the present invention are particularly useful in those applications requiring a pad of low hardness. The composite of the present invention has other desirable physical properties due to the formation of skin layers with good strength and. flexibility. These features render the entire composite dimensionally stable, in spite of the utilization of a bulk or center layer of low cohesive strength and otherwise poor mechanical strength. Furthermore, the properties of the skin layer are such that it eliminates the problem of the bulk layer having a tendency to stick, adhere to, or otherwise block liner surfaces including such typical liner materials as standard silicone release liners and Teflon(copyright), thus rendering materials of the present invention readily removable from the liner after processing or following a particular end use operation or step. Furthermore, the presence of the high strength skin layer makes it possible to easily remove the entire composite from standard silicone release liners. The properties of the skin layer also gives the entire composite sufficient dimensional stability to withstand typical pick-and-place production applications.
The properties of the skin layer permit and make possible the manufacturing and utilization of highly filled and/or low cross-linked materials. In this connection, therefore, the interface pads prepared in accordance with the present invention will flow and otherwise conform to any micro-void or deformity in the surface against which it is placed in the mounting operation. Therefore, interface devices prepared in accordance with the present invention will utilize a skin layer having a greater modulus than the bulk, as well as greater flexibility. This property significantly reduces interfacial thermal resistance and accordingly enhances thermal performance of the entire composite for thermal management purposes.
Therefore, it is a primary object of the present invention to provide an improved interface mounting pad to be interposed between opposed surfaces of a solid state electronic device and a heat dissipating surface, with the pad consisting of a highly filled central bulk layer with low cohesive strength flanked by less highly filled bulk layers having good thermal properties along with excellent mechanical properties, so as to enhance the mechanical strength of the overall composite.
It is yet a further object of the present invention to provide an improved thermal interface pad for use in mounting semiconductor devices to surfaces of a heat sink, with the composite having excellent thermal properties along with good mechanical properties including cohesive strength.